1. Field of the Invention
The present invention relates to an SWR bridge, also known as a microwave impedance bridge, or a directional bridge. More particularly, the present invention relates to components utilized to prevent degradation of connectors on the SWR bridge, and to enable more than one connector type to be used with the SWR bridge.
2. Description of the Related Art
FIG. 1A shows circuitry for a conventional SWR bridge. As shown, the SWR bridge has a first branch including an impedance element 100 having an impedance value Ra. The first branch element 100 is connected to an RF IN input to an SWR bridge to receive the RF input signal. A second branch includes test port 102 which connects a device under test with an arbitrary impedance between the first branch element 100 and ground. A third branch includes an impedance element 104 having an impedance Rb. The third branch element 104 is connected to the RF IN input to receive the RF input signal, similar to element 100. A fourth branch includes an element 106 having a value Rx. The fourth branch element 106 is connected between the third branch element 104 and ground. The SWR bridge of FIG. 1A also includes a balun 108 which provides an impedance of value Re between the branches as shown. The output of balun forms the SWR bridge output, RF OUT, and provides an RF signal proportional to the voltage difference between the connection of elements 100 and 102 to the connection of elements 104 and 106.
FIG. 1B shows another configuration of an SWR bridge which provides a DC output. As shown, the SWR bridge of FIG. 1B replaces the balun 108 of FIG. 1A with a detector 208. Detector 208, like balun 108, provides an impedance Re between branches of the SWR bridge. The output of detector 208 forms the SWR bridge output, DC OUT, and provides a DC signal proportional to the voltage difference between the connection of elements 100 and 102 to the connection of elements 104 and 106.
The SWR bridges of FIGS. 1A and 1B are configured similar to a Wheatstone bridge which is utilized to measure an unknown impedance using signals below microwave frequencies. With a Wheatstone bridge, to determine the impedance of a device under test connected to test port 102, element 106 would be a variable resistance device, while the values of Ra, Rb and Re would be set to a common value Ro, Ro typically being 50.OMEGA.. The impedance of element 106 would be varied until no voltage difference (or a null point) is measured at the SWR bridge output, RF OUT. At the null point, element 106 and a test device connected to test port 102 would have an equal impedance.
For microwave impedance measurements, it has been recognized that the voltage measured at the SWR bridge RF output can be utilized to determine the reflection coefficient, or VSWR of a device under test with the bridge unbalanced, or without a null point being determined. See "Present-Day Simplicity in Broadband SWR Measurements", Wiltron Technical Review, published by Wiltron Company in 1972, pp. 1-3 (hereinafter, 1972 Wiltron publication). In the 1972 Wiltron publication, SWR is calculated from the measured voltage from the SWR bridge output with element 106 being a precision reference termination, and with the element 106 as well as elements 100, 104 and the element with a value Re having a common impedance value Ro.
FIG. 2 shows a block diagram of equipment configured for making scaler measurements with the SWR bridge of FIG. 1A. As shown, the RF IN signal at the RF IN port is provided to the SWR bridge 200 of FIG. 1A from an RF sweep generator 202 over a range of frequencies. The device under test 204 is connected to the test port of the SWR bridge 200, and a diode detector 206 is connected to the output of the SWR bridge. The diode detector 206 converts the AC output of the SWR bridge to a DC voltage value. The DC voltage from the diode detector 206 can then be read from the output of the diode detector 206 using a voltage level meter 208, or a device enabling similar voltage level measurements such as an oscilloscope. Note that the SWR bridge of FIG. 1B can be utilized with a direct connection from the SWR bridge output to the voltage level meter 208 since a diode detector is included in the SWR bridge of FIG. 1B.
One problem with current SWR bridges is the wearing out of the connector on the test port of the SWR bridge. Wear of the test port connector degrades the measurement accuracy of the SWR bridge.
To prevent the connector on the test port of the SWR bridge from wearing out, adapters may be connected between the test port connector of the SWR bridge and devices to be tested. Once an adapter begins to wear, it can then be replaced by another adapter without requiring replacement of the entire microwave component.
Utilizing adapters, however, will degrade the performance of the SWR bridge because the return loss of the adapter is additive to the return loss being measured. For example, a high quality SWR bridge may have 30 dB of directivity, but adapters may drop the directivity to 20 dB. Some components to be measured may have a return loss below 20 dB, making utilization of an adapter impractical.
Another problem with current SWR bridges is the multiplicity of connector types which may be utilized at the test port of the SWR bridge to mate with a connector of a device to be tested. For example, some connectors which may be utilized at the test port include SMA, 3.5 mm, 2.92 mm, 2.5 mm and 1.85 mm connectors. Further, each of these connectors include male and female versions, creating a selection of ten possible connectors for use on the test port of an SWR bridge with the connector types indicated above. The different connectors may not mechanically mate, and if they do mechanically mate, an electrical mismatch will result. Utilizing an adapter between the test port connector and the connector of a device under test to provide mechanical and/or electrical compatibility may unacceptably degrade the performance of the SWR bridge, as indicated above. Thus, a user may need to acquire a separate SWR bridge for each type of connector.
One way to prevent the need for a different SWR bridge for each connector type without significantly degrading performance is to measure SWR of a device through the SWR bridge using a vector network analyzer (VNA), instead of using a detector and voltmeter to provide scaler measurements. The VNA can then be programmed or calibrated to cancel out the mismatch of any adapter. See Daw, "Improved Accuracy of Vector Measurements for SMA Components", Microwave and RF, August 1988; Pollard, "Compensation Technique Improves Measurements for a Range of Mechanically Compatible Connectors", Microwave Journal, October 1994, pp. 91-99.
However, an SWR bridge configured to provide measurements in a scaler mode may still be desirable over vector measurements using a VNA. First, the SWR bridge is typically utilized as a lower cost alternative to the VNA. Additionally, the SWR bridge operating in a scaler mode provides more rapid measurements as opposed to vector mode, the faster measurements being desirable, for instance, during tuning. Further, calibration software for the VNA to cancel out the mismatch of an adapter is cumbersome, and will require a new calibration for each different connector type utilized with the SWR bridge.